Most television tuners have means to interrupt the automatic fine tuning (AFT) circuit of a television tuner during periods when the selector shaft of the tuner is moved from one channel to another. This is necessary because transient frequency excursions during commutation, and residual AFT energy from the channel being abandoned, can cause undesirable interactions between the AFT circuit and the local oscillator which sometimes results in a so-called AFT lock out condition wherein the operation of the automatic fine tuning circuit captures the local oscillator and maintains that a stable frequency which is, however, not the correct one for the channel selected. Also, for filtering and circuit stability reasons, the AFT voltage source is normally heavily by passed with a large capacitor so that the AFT control voltage remains at the level required for the channel being abandoned upon reaching an adjacent channel which has been selected. Unless discharged before reaching the adjacent channel, the AFT charge potential may cause momentary detuning of the oscillator at this new channel shifting it to regions resulting in the reception of undesired signals on which the AFT circuit may become locked.
To prevent the aforementioned malfunctioning during changing of channels, it is common practice to provide automatic means to momentarily suppress the AFT action while channels are being changed. This is generally accomplished by a switch, sometimes called an AFT defeat switch, which is mechanically actuated between channel positions and is effective in placing the AFT circuit in its quiescent state and in preventing it from controlling the local oscillator until the desired channel commutation has been completed. This AFT defeat switch is generally actuated by cams or linkages that sense the position of mechanical elements in the tuner, such as the channel detent means, to cause AFT disabling action between channel positions. Electrically, the AFT suppression is accomplished by momentarily connecting the AFT control line to a reference source of voltage by means of the AFT defeat switch. When the desired channel has been selected and the detent means is seated, the AFT switch disabling function ceases and the AFT circuit is enabled to perform the function of keeping the local oscillator on the correct frequency, as long as the selected channel is not changed.
The AFT defeat switches heretofore used have been external to the tuner housing and have been controlled by followers, actuators and the like which sense the position of the mechanical elements involved in channel changing, such as the channel detent means, these followers or actuators functioning to cause the switch to close or to open at the appropriate time. Since these switches are external to the tuner shaft they require wiring interconnection with the internal circuits of the tuner associated with the AFT function. These mechanical elements which control the disabling and re-enabling of the AFT circuit must be accurately synchronized with the channel changing operation so that automatic frequency control of the oscillator is reestablished only when the frequency of the oscillator has stabilized upon reaching the detent seated position. The normal dimensional variations in mass produced mechanical elements, such actuating cams, levers or followers, makes the inclusion of adjusting means on the switch actuating train mandatory, thereby adding to the complexity and cost of the system as well as requiring skilled personnel for the adjustments required to provide adequate synchronization. Additionally, the wiring interconnection between the internal local oscillator circuit and the external switch can cause leakage of high frequency energy from the oscillator which produces undesirable interference with receivers in the vicinity of the local oscillator.
Examples of such prior art AFT defeat mechanisms are shown in Chipman U.S. Pat. No. 3,328,700, Milnes U.S. Pat. No. 3,466,549, Tap U.S. Pat. No. 3,798,579, Susuki U.S. Pat. No. 3,870,961, Valdettaro U.S. Pat. No. 3,842,683, Badger U.S. Pat. No. 3,906,805, and Valdettaro U.S. Pat. No. 3,924,209.